1. Field of the Invention
This invention pertains to an apparatus and a process for forming a printing form from a photosensitive element and, in particular, to an apparatus and a process for forming the printing form by a thermal development process and, more particularly, to an apparatus and a process for thermally developing a photosensitive element having a cylindrical support.
2. Description of Related Art
Flexographic printing plates are well known for use in relief printing on a variety of substrates such as paper, corrugated board, films, foils and laminates. Flexographic printing plates can be prepared from photosensitive elements containing a layer of a photosensitive composition such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. Photosensitive compositions, which may be referred to as photopolymerizable compositions, generally contain an elastomeric binder, at least one monomer, and a photoinitiator. Photosensitive elements generally have the layer of the photopolymerizable composition interposed between a support and a cover sheet or multilayer cover element. Upon imagewise exposure of the photosensitive element to actinic radiation, photopolymerization of the photosensitive composition occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution, e.g., solvent or solvent mixture or aqueous-based solution, to remove areas of the photopolymerizable layer that were not exposed and leaving a printing relief which can be used for flexographic printing.
However, developing systems that treat the element with a solution are time consuming since drying for extended period (0.5 to 24 hours) is necessary to remove developer solution entrained in the element. In addition, developing systems that use solvent solutions also can produce volatile organic vapor as well as potentially harmful by-products as waste (both the solvent and any material carried off by the solvent) during development.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the composition layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. Nos. 3,060,023 (Burg et al.); 3,264,103 (Cohen et al.); 5,015,556 (Martens); 5,175,072 (Martens); 5,215,859 (Martens) and 5,279,697 (Peterson et al.). The exposed portions of the composition layer remain hard, that is, do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated and/or removed from the composition layer. The cycle of heating and contacting the composition layer may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. Thus remains a raised relief structure of irradiated, hardened composition that represents the desired printing image.
Processors for thermal development of flexographic printing elements are known. U.S. Pat. No. 5,279,697 (Peterson et al.) describes an automated process and apparatus for heat treating an imagewise irradiated photosensitive sheet to remove unwanted portions and leave radiation hardened relief images on the sheet. PCT publication WO 2001/18604 also describes a method and apparatus for thermal processing a photosensitive element. In both thermal processing apparatuses an irradiated photosensitive printing element comprising the support and the composition layer is mounted on a drum and a continuous web of absorbent material is passed over a hot roll. The hot roll is urged towards the drum pressing the web against the photosensitive element and forming a nip. The hot roll may be heated by an electrical core heater or by other means to provide a temperature sufficient to melt a portion of the composition on the flexible film support. Heat is transferred by conduction from the hot roll, through the absorbent web, to the photosensitive element upon contact so the temperature of the composition layer is raised sufficiently to enable the unirradiated portions of the composition layer to liquefy and be absorbed into the absorbent material. As the drum and hot roll rotate in contact together, the web is pressed against the photosensitive element to absorb the liquefied unirradiated composition and is then separated from the element. Several cycles of passing the element past the hot roll are repeated to progressively remove the unirradiated composition from the printing element. After the unirradiated composition is removed the resulting element has a raised relief surface of hardened areas that is suitable use as a printing plate.
U.S. Pat. No. 5,279,697 describes heating the drum to preheat the composition layer to a temperature near the melt point of the unirradiated areas. But the heated drum may heat the support of the element to such an extent that the support may distort and/or shrink which can affect the dimensional stability of the relief area of the printing plate. WO 01/18604 avoids such undesirable distortion and/or shrinkage of the support by cooling the support simultaneous to the heating of the composition layer. Cooling of the support is carried out by blowing air on the circumferential surface of the drum while providing additional heat to an exterior surface of the composition layer with infrared heating devices.
Up to now, flat or planar photosensitive printing elements were typically processed in commercial thermal development processors. It was also possible to thermally process cylindrically-shaped photosensitive printing elements, such as seamless photopolymer sleeves, or so called plate-on-sleeves. Seamless photopolymer sleeves include at least a continuous or substantially continuous layer of the photopolymerizable composition on a cylindrical support. Plate-on-sleeves include a flat photosensitive printing element mounted onto a cylindrical support. But for each diameter of the sleeve possible for cylindrical printing elements either an extra support cylinder, i.e., drum or airshaft, needed to be fitted into the thermal processor, or the support cylinder needed to be fitted with a special adapter sleeve or sleeves. Replacement of the support cylinder in the thermal processor to accommodate different diameter sleeves is impractical since this would involve considerable downtime in the operation of the processor and cost in tooling for the processor.
So, thermal development of cylindrical photosensitive printing elements on sleeves with different diameters on commercial processors is difficult, time consuming, and expensive.